This invention relates generally to the field of photocells and photo-imagers, and more particularly to multi-color photocells and imagers.
Photocells and photo-imagers (also called xe2x80x9cimage sensorsxe2x80x9d) made of an array of photocells are used in a wide variety of imaging applications. One type of photo-imager is a CCD (charge-coupled device), in which each photocell is normally sensitive to only one band of wavelengths (i.e., one color). A conventional CCD imager can be complex and require highly accurate timing mechanisms and is therefore expensive to manufacture. Conventional CCDs also require 5-10 volt supplies and are limited in detecting lower-wavelength (e.g., blue light) signals.
One type of imager that operates at a lower voltage, is simpler, and is less expensive to manufacture than the CCD imager is a metal-insulator-semiconductor (xe2x80x9cM-I-S,xe2x80x9d also called xe2x80x9cmetal-insulator-siliconxe2x80x9d) imager. An M-I-S imager is made of an array of M-I-S photocells, one of which is shown in a schematic diagram in FIG. 1A. The M-I-S photocell shown in FIG. 1A is derived from the applicants"" prior unexamined Japanese patent application JP 10-27896, published Jan. 27, 1998, the disclosure of which is herein incorporated by reference. As the name implies, the M-I-S photocell comprises a metal contact or electrode 120 (also called a gate), an insulator 130, and a semiconductor diffusion layer 140. When a negative drive voltage is applied to diffusion layer 140, a depletion region 150 is formed within diffusion layer 140 in the area below contact 120. This M-I-S structure 100 is disposed on top of a substrate 10, which is typically made from silicon. Insulator 130 is typically made of a thermally grown oxide such as SiO2. When thin, between 20 and 50 xc3x85, typically 30 xc3x85, insulator 130 acts as a dielectric material and is light-transparent. Contact 120 is also light-transparent, typically made of a metal, such as SnO2, or heavily doped polysilicon, and has a typical thickness of 2000 xc3x85, when made of polysilicon. The resistivity of contact 120 varies depending on the material: for polysilicon, it has a resistivity of 100xcexa9-200xcexa9 per square; for SnO2, it has a resistivity of approximately 10xcexa9-20xcexa9 per square. When substrate 10 comprises n-Si, then diffusion layer 140 comprises single-crystal p-Si, and contact 120 is an n-contact or a p-contact. The photocell generates current when light transmits through contact 120 and insulator 130 and is incident on depletion region 150 creating electron-hole pairs.
The M-I-S photocell can be arranged in an array to form an M-I-S photo-imager, a portion of which is shown in FIG. 1B. Semiconductor diffusion layers 140 are arranged in strips over substrate 10. Insulator 130 is disposed over diffusion layers 140. Between the diffusion layers are isolation strips 160 made by depositing more thickly the insulator material directly on substrate 10. (These strips 160 may also be made using a LOCOS (local oxidization of silicon) process.) Perpendicular to the diffusion layers and isolation strips are strips of contact 120. The depletion regions 150 are formed at the intersections of the contact 120 and the diffusion layers 140.
The quantum efficiency of each photocell with a 2000 xc3x85 polysilicon contact layer is shown in TABLE 1 as a function of color and wavelength.
Although providing a significant improvement over CCD imagers with respect to power dissipation and the detection of some colors, this M-I-S photo-imager with a 2000 xc3x85 polysilicon contact does not detect blue light very well, as demonstrated in TABLE 1, and is not able to separate out more than one color.
Therefore, a need has arisen for an improved photo-imager which is capable of detecting several colors, including blue light. In accordance with the present invention, a device, such as a photocell, for detecting light includes at least two structures or tiers, one disposed over the other, each detecting a different wavelength of light. Each structure could be an M-I-S structure or a semiconductor-insulator-metal (S-I-M) structure.
Preferably, each M-I-S structure includes a semiconductor diffusion layer capable of developing a depletion region, a thin insulator layer disposed on the diffusion layer, and a contact layer disposed on the thin insulator layer. Each S-I-M structure includes a contact layer, a thin insulator layer disposed on the contact layer, and a semiconductor diffusion layer disposed on the thin insulator layer, the diffusion layer capable of developing a depletion region. When light is incident on each M-I-S or S-I-M depletion region, a current indicative of the light detected in each M-I-S or S-I-M structure flows through the respective contact layer.
A photo-imager according to the present invention includes an array of photocells each photocell including the two M-I-S or S-I-M structures.
Preferably, the wavelength detected by the bottom tier is longer than the wavelength detected by the top tier.
Preferably, a third M-I-S or S-I-M structure (or tier) is disposed over the first two tiers, and all three tiers detect different wavelengths. Preferably, the third tier has a metal-insulator-semiconductor or semiconductor-insulator-metal structure analogous to those of the first two tiers.
Preferably, the top, middle, and bottom tiers of the three-tiered device detect light having progressively longer wavelengths. Preferably, the top tier detects mainly blue light, the middle tier mainly green light, and the bottom tier mainly red light.
Preferred embodiments of this three-tiered device include all M-I-S structures, all S-I-M structures, an M-I-S/S-I-M/M-I-S (in order from top to bottom) structure, an S-I-M/M-I-S/S-I-M structure, and an S-I-M/S-I-M/M-I-S structure.
Also in accordance with the present invention is a device, such as a photocell, for detecting light that includes at least one M-I-S structure. The M-I-S structure includes a semiconductor diffusion layer, a thin insulator layer disposed on the diffusion layer, and a contact layer disposed on the thin insulator layer. The diffusion layer is made of polysilicon or amorphous silicon and is capable of developing a depletion region. When light is incident on the depletion region, a current indicative of the light detected in the depletion region flows through the contact layer. Preferably, a photo-imager according to the present invention includes an array of photocells each having such an M-I-S structure.
Also in accordance with the present invention is a device, such as a photocell, for detecting light that includes an S-I-M structure. The S-I-M structure includes a contact layer, a thin insulator layer disposed on the contact layer, and a semiconductor diffusion layer disposed on the thin insulator layer. The diffusion layer is capable of developing a depletion region. When light is incident on the depletion region, a current indicative of the light detected in the depletion region flows through the contact layer. Preferably, a photo-imager according to the present invention includes an array of photocells each having an S-I-M structure. The diffusion layer can be made of polysilicon or amorphous silicon.
Also in accordance with the present invention are methods for fabricating a device, such as a photocell, for detecting light and a photo-imager comprising an array of those photocells. One method includes forming on a substrate three tiers or structures, one disposed over the next, each detecting a different wavelength of light. Each tier could be an M-I-S or an S-I-M structure. Preferably, the top, middle, and bottom tiers detect light having progressively longer wavelengths. Preferably, the top tier detects mainly blue light, the middle tier mainly green light, and the bottom tier mainly red light.
A second method includes fabricating a photocell device on a substrate based on an S-I-M structure. The S-I-M structure is fabricated by forming a contact layer on the substrate, forming a thin insulator layer on the contact layer, and forming a semiconductor diffusion layer on the thin insulator layer, the diffusion layer capable of developing a depletion region. When light is incident on the depletion region, a current indicative of the light detected in the depletion region flows through the contact layer.
The present invention provides various advantages. One advantage is that the three-tiered devices detect three main colors, and do so relatively inexpensively and at a lower voltage compared with conventional devices. Another advantage is that the S-I-M structure and the two-and three-tiered devices are able to detect blue light better than conventional devices.
Other technical advantages of the present invention will be readily apparent to one skilled in the art from the following figures, description, and claims.